Chapter 3

Mutations and Health

Table of Contents3What is a gene mutation and how do mutations occur?

 5How can gene mutations affect health and development?

 6Do all gene mutations affect health and development?

 7What kinds of gene mutations are possible?

 12Can a change in the number of genes affect health and development?

 13Can changes in the number of chromosomes affect health and

development? 

20Can changes in the structure of chromosomes affect health anddevelopment? 

30Can changes in mitochondrial DNA affect health and development? 

32What are complex or multifactorial disorders? 

33What information about a genetic condition can statistics provide? 

36How are genetic conditions and genes named? 

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What is a gene mutation and how do mutations occur?A gene mutation is a permanent change in the DNA sequence that makes up agene. Mutations range in size from a single DNA building block (DNA base) to alarge segment of a chromosome.

Gene mutations occur in two ways: they can be inherited from a parent or acquiredduring a person’s lifetime. Mutations that are passed from parent to child are calledhereditary mutations or germline mutations (because they are present in the eggand sperm cells, which are also called germ cells). This type of mutation is presentthroughout a person’s life in virtually every cell in the body.

Mutations that occur only in an egg or sperm cell, or those that occur just afterfertilization, are called new (de novo) mutations. De novo mutations may explaingenetic disorders in which an affected child has a mutation in every cell, but hasno family history of the disorder.

Acquired (or somatic) mutations occur in the DNA of individual cells at some timeduring a person’s life. These changes can be caused by environmental factors suchas ultraviolet radiation from the sun, or can occur if a mistake is made as DNAcopies itself during cell division. Acquired mutations in somatic cells (cells otherthan sperm and egg cells) cannot be passed on to the next generation.

Mutations may also occur in a single cell within an early embryo. As all the cellsdivide during growth and development, the individual will have some cells with themutation and some cells without the genetic change. This situation is calledmosaicism.

Some genetic changes are very rare; others are common in the population. Geneticchanges that occur in more than 1 percent of the population are calledpolymorphisms. They are common enough to be considered a normal variation inthe DNA. Polymorphisms are responsible for many of the normal differences betweenpeople such as eye color, hair color, and blood type. Although many polymorphismshave no negative effects on a person’s health, some of these variations mayinfluence the risk of developing certain disorders.

For more information about mutations:

The National Cancer Institute offers a discussion of hereditary mutations(http://www.cancer.gov/cancertopics/understandingcancer/genetesting/page11)and information about acquired mutations (http://www.cancer.gov/cancertopics/understandingcancer/genetesting/page12).

The Centre for Genetics Education provides a fact sheet discussing changes to thegenetic code (http://www.genetics.edu.au/Information/Genetics-Fact-Sheets/Changes-to-the-Genetic-Code-FS4).

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How can gene mutations affect health and development?To function correctly, each cell depends on thousands of proteins to do their jobsin the right places at the right times. Sometimes, gene mutations prevent one ormore of these proteins from working properly. By changing a gene’s instructionsfor making a protein, a mutation can cause the protein to malfunction or to bemissing entirely. When a mutation alters a protein that plays a critical role in thebody, it can disrupt normal development or cause a medical condition. A conditioncaused by mutations in one or more genes is called a genetic disorder.

In some cases, gene mutations are so severe that they prevent an embryo fromsurviving until birth. These changes occur in genes that are essential fordevelopment, and often disrupt the development of an embryo in its earliest stages.Because these mutations have very serious effects, they are incompatible with life.

It is important to note that genes themselves do not cause disease—geneticdisorders are caused by mutations that make a gene function improperly. Forexample, when people say that someone has “the cystic fibrosis gene,” they areusually referring to a mutated version of the CFTR gene, which causes the disease.All people, including those without cystic fibrosis, have a version of the CFTR gene.

For more information about mutations and genetic disorders:

The National Cancer Institute provides additional information about how genemutations can trigger disease:

• Gene Mutations and Disease (http://www.cancer.gov/cancertopics/understandingcancer/genetesting/page8)

• Altered DNA, Altered Protein (http://www.cancer.gov/cancertopics/understandingcancer/genetesting/page10)

The Centre for Genetics Education offers a fact sheet about genetic changes thatlead to disorders (http://www.genetics.edu.au/Information/Genetics-Fact-Sheets/Changes-that-make-a-gene-faulty-FS5).

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Do all gene mutations affect health and development?No; only a small percentage of mutations cause genetic disorders—most have noimpact on health or development. For example, some mutations alter a gene’s DNAsequence but do not change the function of the protein made by the gene.

Often, gene mutations that could cause a genetic disorder are repaired by certainenzymes before the gene is expressed and an altered protein is produced. Eachcell has a number of pathways through which enzymes recognize and repairmistakes in DNA. Because DNA can be damaged or mutated in many ways, DNArepair is an important process by which the body protects itself from disease.

A very small percentage of all mutations actually have a positive effect. Thesemutations lead to new versions of proteins that help an individual better adapt tochanges in his or her environment. For example, a beneficial mutation could resultin a protein that protects an individual and future generations from a new strain ofbacteria.

Because a person’s genetic code can have a large number of mutations with noeffect on health, diagnosing genetic conditions can be difficult. Sometimes, genesthought to be related to a particular genetic condition have mutations, but whetherthese changes are involved in development of the condition has not beendetermined; these genetic changes are known as variants of unknown significance(VOUS). Sometimes, no mutations are found in suspected disease-related genes,but mutations are found in other genes whose relationship to a particular geneticcondition is unknown. It is difficult to know whether these variants are involved inthe disease.

For more information about DNA repair and the health effects of gene mutations:

The University of Utah Genetic Science Learning Center provides information aboutgenetic disorders (http://learn.genetics.utah.edu/content/disorders/whataregd/) thatexplains why some mutations cause disorders but others do not.

Additional information about DNA repair is available from the National Center forBiotechnology Information’s (NCBI) Science Primer. In the chapter called What IsA Cell? (http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html), scroll downto the heading “DNA Repair Mechanisms.”

The Yale Cancer Center describes the implications of finding variants of unknownsignificance (VOUS) (http://medicine.yale.edu/cancer/patient/support/genetics/88717_Information_about_Variants_of_Uncertain_Significance-1-2011.pdf). Theinformation applies to VOUS in cancer and other genetic conditions.

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What kinds of gene mutations are possible?The DNA sequence of a gene can be altered in a number of ways. Gene mutationshave varying effects on health, depending on where they occur and whether theyalter the function of essential proteins. The types of mutations include:

Missense mutation (illustration on page 8)

This type of mutation is a change in one DNA base pair that results in thesubstitution of one amino acid for another in the protein made by a gene.

Nonsense mutation (illustration on page 9)

A nonsense mutation is also a change in one DNA base pair. Instead ofsubstituting one amino acid for another, however, the altered DNA sequenceprematurely signals the cell to stop building a protein. This type of mutationresults in a shortened protein that may function improperly or not at all.

Insertion (illustration on page 9)

An insertion changes the number of DNA bases in a gene by adding a pieceof DNA. As a result, the protein made by the gene may not function properly.

Deletion (illustration on page 10)

A deletion changes the number of DNA bases by removing a piece of DNA.Small deletions may remove one or a few base pairs within a gene, whilelarger deletions can remove an entire gene or several neighboring genes. Thedeleted DNA may alter the function of the resulting protein(s).

Duplication (illustration on page 10)

A duplication consists of a piece of DNA that is abnormally copied one or moretimes. This type of mutation may alter the function of the resulting protein.

Frameshift mutation (illustration on page 11)

This type of mutation occurs when the addition or loss of DNA bases changesa gene’s reading frame. A reading frame consists of groups of 3 bases thateach code for one amino acid. A frameshift mutation shifts the grouping ofthese bases and changes the code for amino acids. The resulting protein isusually nonfunctional. Insertions, deletions, and duplications can all beframeshift mutations.

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Repeat expansion (illustration on page 11)

Nucleotide repeats are short DNA sequences that are repeated a number oftimes in a row. For example, a trinucleotide repeat is made up of 3-base-pairsequences, and a tetranucleotide repeat is made up of 4-base-pair sequences.A repeat expansion is a mutation that increases the number of times that theshort DNA sequence is repeated. This type of mutation can cause the resultingprotein to function improperly.

For more information about the types of gene mutations:

The National Human Genome Research Institute offers a Talking Glossary ofGenetic Terms (http://www.genome.gov/Glossary/). This resource includesdefinitions, diagrams, and detailed audio descriptions of several of the genemutations listed above.

Illustrations

In this example, the nucleotide adenine is replaced by cytosine in the geneticcode, introducing an incorrect amino acid into the protein sequence.

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In this example, the nucleotide cytosine is replaced by thymine in the DNAcode, signaling the cell to shorten the protein.

In this example, one nucleotide (adenine) is added in the DNA code, changingthe amino acid sequence that follows.

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In this example, one nucleotide (adenine) is deleted from the DNA code,changing the amino acid sequence that follows.

A section of DNA is accidentally duplicated when a chromosome is copied.

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A frameshift mutation changes the amino acid sequence from the site of themutation.

In this example, a repeated trinucleotide sequence (CAG) adds a series of theamino acid glutamine to the resulting protein.

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Can a change in the number of genes affect health anddevelopment?

People have two copies of most genes, one copy inherited from each parent. Insome cases, however, the number of copies varies—meaning that a person canbe born with one, three, or more copies of particular genes. Less commonly, oneor more genes may be entirely missing. This type of genetic difference is knownas copy number variation (CNV).

Copy number variation results from insertions, deletions, and duplications of largesegments of DNA. These segments are big enough to include whole genes. Variationin gene copy number can influence the activity of genes and ultimately affect manybody functions.

Researchers were surprised to learn that copy number variation accounts for asignificant amount of genetic difference between people. More than 10 percent ofhuman DNA appears to contain these differences in gene copy number. Whilemuch of this variation does not affect health or development, some differenceslikely influence a person’s risk of disease and response to certain drugs. Futureresearch will focus on the consequences of copy number variation in different partsof the genome and study the contribution of these variations to many types ofdisease.

For more information about copy number variation:

The Howard Hughes Medical Institute discusses the results of recent research oncopy number variation in the news release, Genetic Variation: We’re More DifferentThan We Thought (http://www.hhmi.org/news/scherer20061123.html).

For people interested in more technical data, several institutions provide databasesof structural differences in human DNA, including copy number variation:

• Database of Genomic Variants (http://projects.tcag.ca/variation/)

• The Sanger Institute: Database of Chromosomal Imbalance andPhenotype in Humans using Ensembl Resources (DECIPHER(http://decipher.sanger.ac.uk/))

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Can changes in the number of chromosomes affect healthand development?

Human cells normally contain 23 pairs of chromosomes, for a total of 46chromosomes in each cell (illustration on page 15). A change in the number ofchromosomes can cause problems with growth, development, and function of thebody’s systems. These changes can occur during the formation of reproductivecells (eggs and sperm), in early fetal development, or in any cell after birth. A gainor loss of chromosomes from the normal 46 is called aneuploidy.

A common form of aneuploidy is trisomy, or the presence of an extra chromosomein cells. “Tri-” is Greek for “three”; people with trisomy have three copies of aparticular chromosome in cells instead of the normal two copies. Down syndromeis an example of a condition caused by trisomy (illustration on page 16).  Peoplewith Down syndrome typically have three copies of chromosome 21 in each cell,for a total of 47 chromosomes per cell.

Monosomy, or the loss of one chromosome in cells, is another kind of aneuploidy.“Mono-” is Greek for “one”; people with monosomy have one copy of a particularchromosome in cells instead of the normal two copies. Turner syndrome is acondition caused by monosomy (illustration on page 17). Women with Turnersyndrome usually have only one copy of the X chromosome in every cell, for a totalof 45 chromosomes per cell.

Rarely, some cells end up with complete extra sets of chromosomes.  Cells withone additional set of chromosomes, for a total of 69 chromosomes, are called triploid(illustration on page 18).  Cells with two additional sets of chromosomes, for a totalof 92 chromosomes, are called tetraploid.  A condition in which every cell in thebody has an extra set of chromosomes is not compatible with life.

In some cases, a change in the number of chromosomes occurs only in certaincells.  When an individual has two or more cell populations with a differentchromosomal makeup, this situation is called chromosomal mosaicism (illustrationon page 19).  Chromosomal mosaicism occurs from an error in cell division in cellsother than eggs and sperm. Most commonly, some cells end up with one extra ormissing chromosome (for a total of 45 or 47 chromosomes per cell), while othercells have the usual 46 chromosomes. Mosaic Turner syndrome is one exampleof chromosomal mosaicism.  In females with this condition, some cells have 45chromosomes because they are missing one copy of the X chromosome, whileother cells have the usual number of chromosomes.

Many cancer cells also have changes in their number of chromosomes. Thesechanges are not inherited; they occur in somatic cells (cells other than eggs orsperm) during the formation or progression of a cancerous tumor.

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For more information about chromosomal disorders:

A discussion of how chromosomal abnormalities happen (http://www.genome.gov/11508982#6) is provided by the National Human Genome Research Institute.

The Centre for Genetics Education offers a fact sheet about changes in chromosomenumber or size (http://www.genetics.edu.au/Information/Genetics-Fact-Sheets/Changes-to-Chromosomes-Number-Size-and-Structure-FS6).

The National Organization for Rare Disorders offers an overview of triploid syndrome(http://www.rarediseases.org/rare-disease-information/rare-diseases/byID/710/viewAbstract).

Chromosomal Mosaicism (http://mosaicism.cfri.ca/index.htm), a web site providedby the University of British Columbia, offers detailed information about mosaicchromosomal abnormalities.

MedlinePlus offers an encyclopedia article about chromosomal mosaicism(http://www.nlm.nih.gov/medlineplus/ency/article/001317.htm).

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Illustrations

Human cells normally contain 23 pairs of chromosomes, for a total of 46chromosomes in each cell.

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Trisomy is the presence of an extra chromosome in cells. Down syndrome isan example of a condition caused by trisomy.

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Monosomy is the loss of one chromosome in cells. Turner syndrome is anexample of a condition caused by monosomy.

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Cells with one additional set of chromosomes, for a total of 69 chromosomes,are called triploid.

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When an individual has two or more cell populations with a differentchromosomal makeup, this situation is called chromosomal mosaicism.

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Can changes in the structure of chromosomes affecthealth and development?

Changes that affect the structure of chromosomes can cause problems with growth,development, and function of the body’s systems. These changes can affect manygenes along the chromosome and disrupt the proteins made from those genes.

Structural changes can occur during the formation of egg or sperm cells, in earlyfetal development, or in any cell after birth. Pieces of DNA can be rearranged withinone chromosome or transferred between two or more chromosomes. The effectsof structural changes depend on their size and location, and whether any geneticmaterial is gained or lost.  Some changes cause medical problems, while othersmay have no effect on a person’s health.

Changes in chromosome structure include:

Translocations (illustration: balanced on page 22),(illustration: unbalanced on page 23)

A translocation occurs when a piece of one chromosome breaks off andattaches to another chromosome.  This type of rearrangement is describedas balanced if no genetic material is gained or lost in the cell.  If there is again or loss of genetic material, the translocation is described as unbalanced. 

Deletions (illustration on page 24)

Deletions occur when a chromosome breaks and some genetic material islost.  Deletions can be large or small, and can occur anywhere along achromosome.

Duplications (illustration on page 25)

Duplications occur when part of a chromosome is copied (duplicated) toomany times.  This type of chromosomal change results in extra copies ofgenetic material from the duplicated segment.

Inversions (illustration on page 26)

An inversion involves the breakage of a chromosome in two places; theresulting piece of DNA is reversed and re-inserted into the chromosome. Genetic material may or may not be lost as a result of the chromosome breaks. An inversion that involves the chromosome’s constriction point (centromere)is called a pericentric inversion.  An inversion that occurs in the long (q) armor short (p) arm and does not involve the centromere is called a paracentricinversion.

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Isochromosomes (illustration on page 27)

An isochromosome is a chromosome with two identical arms.  Instead of onelong (q) arm and one short (p) arm, an isochromosome has two long arms ortwo short arms.  As a result, these abnormal chromosomes have an extracopy of some genes and are missing copies of other genes.

Dicentric chromosomes (illustration on page 28)

Unlike normal chromosomes, which have a single constriction point(centromere), a dicentric chromosome contains two centromeres.  Dicentricchromosomes result from the abnormal fusion of two chromosome pieces,each of which includes a centromere.  These structures are unstable and ofteninvolve a loss of some genetic material.

Ring chromosomes (illustration on page 29)

Ring chromosomes usually occur when a chromosome breaks in two placesand the ends of the chromosome arms fuse together to form a circularstructure.  The ring may or may not include the chromosome’s constrictionpoint (centromere).  In many cases, genetic material near the ends of thechromosome is lost.

Many cancer cells also have changes in their chromosome structure. These changesare not inherited; they occur in somatic cells (cells other than eggs or sperm) duringthe formation or progression of a cancerous tumor.

For more information about structural changes to chromosomes:

The National Human Genome Research Institute provides a list of questions andanswers about chromosome abnormalities (http://www.genome.gov/11508982),including a glossary of related terms.

Chromosome Deletion Outreach offers a fact sheet on this topic titled Introductionto Chromosomes (http://www.chromodisorder.org/CDO/General/IntroToChromosomes.aspx). This resource includes illustrated explanations ofseveral chromosome abnormalities.

The Centre for Genetics Education provides fact sheets about changes inchromosome number or size (http://www.genetics.edu.au/Information/Genetics-Fact-Sheets/Changes-to-Chromosomes-Number-Size-and-Structure-FS6) andchromosomal rearrangements (translocations) (http://www.genetics.edu.au/Information/Genetics-Fact-Sheets/Changes-to-Chromosome-Structure-Translocations-FS7).

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More technical information is available from the textbook Human Molecular Genetics(second edition, 1999) in the section about structural chromosome abnormalities(http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=hmg.section.196#209).

The Atlas of Genetics and Cytogenetics in Oncology and Haematology provides atechnical introduction to chromosomal aberrations (http://atlasgeneticsoncology.org/Deep/Chromaber.html) and a detailed discussion of ring chromosomes(http://atlasgeneticsoncology.org/Deep/RingChromosID20030.html), particularlytheir role in cancer.

Illustrations

In a balanced translocation, pieces of chromosomes are rearranged but nogenetic material is gained or lost in the cell.

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An unbalanced translocation occurs when a child inherits a chromosome withextra or missing genetic material from a parent with a balanced translocation.

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A deletion occurs when a chromosome breaks and some genetic material islost. 

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A duplication occurs when part of a chromosome is copied (duplicated)abnormally, resulting in extra genetic material from the duplicated segment.

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Inversions occur when a chromosome breaks in two places and the resultingpiece of DNA is reversed and re-inserted into the chromosome. Inversions that

involve the centromere are called pericentric inversions; those that do notinvolve the centromere are called paracentric inversions.

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An isochromosome is an abnormal chromosome with two identical arms, eithertwo short (p) arms or two long (q) arms.

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Dicentric chromosomes result from the abnormal fusion of two chromosomepieces, each of which includes a centromere. 

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Ring chromosomes usually occur when a chromosome breaks in two placesand the ends of the chromosome arms fuse together to form a circular structure.

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Can changes in mitochondrial DNA affect health anddevelopment?

Mitochondria (illustration on page 31) are structures within cells that convert theenergy from food into a form that cells can use. Although most DNA is packagedin chromosomes within the nucleus, mitochondria also have a small amount of theirown DNA (known as mitochondrial DNA or mtDNA). In some cases, inheritedchanges in mitochondrial DNA can cause problems with growth, development, andfunction of the body’s systems. These mutations disrupt the mitochondria’s abilityto generate energy efficiently for the cell.

Conditions caused by mutations in mitochondrial DNA often involve multiple organsystems. The effects of these conditions are most pronounced in organs and tissuesthat require a lot of energy (such as the heart, brain, and muscles). Although thehealth consequences of inherited mitochondrial DNA mutations vary widely,frequently observed features include muscle weakness and wasting, problems withmovement, diabetes, kidney failure, heart disease, loss of intellectual functions(dementia), hearing loss, and abnormalities involving the eyes and vision.

Mitochondrial DNA is also prone to somatic mutations, which are not inherited.Somatic mutations occur in the DNA of certain cells during a person’s lifetime andtypically are not passed to future generations. Because mitochondrial DNA has alimited ability to repair itself when it is damaged, these mutations tend to build upover time. A buildup of somatic mutations in mitochondrial DNA has been associatedwith some forms of cancer and an increased risk of certain age-related disorderssuch as heart disease, Alzheimer disease, and Parkinson disease. Additionally,research suggests that the progressive accumulation of these mutations over aperson’s lifetime may play a role in the normal process of aging.

For more information about conditions caused by mitochondrial DNA mutations:

Genetics Home Reference provides background information about mitochondriaand mitochondrial DNA (http://ghr.nlm.nih.gov/handbook/basics/mtdna) written inconsumer-friendly language.

The Cleveland Clinic offers a basic introduction to mitochondrial disease(http://my.clevelandclinic.org/disorders/Mitochondrial_Disease/hic_Myths_and_Facts_About_Mitochondrial_Diseases.aspx).

An overview of mitochondrial disorders (http://www.ncbi.nlm.nih.gov/books/NBK1224/) is available from GeneReviews.

The Muscular Dystrophy Association offers an introduction to mitochondrial disordersas part of their fact sheet called Mitochondrial Myopathies (http://mda.org/disease/mitochondrial-myopathies).

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The Neuromuscular Disease Center at Washington University provides an in-depthdescription of many mitochondrial conditions (http://neuromuscular.wustl.edu/mitosyn.html).

Illustrations

Mitochondria provide the cell’s energy.

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What are complex or multifactorial disorders?Researchers are learning that nearly all conditions and diseases have a geneticcomponent. Some disorders, such as sickle cell anemia and cystic fibrosis, arecaused by mutations in a single gene. The causes of many other disorders, however,are much more complex. Common medical problems such as heart disease,diabetes, and obesity do not have a single genetic cause—they are likely associatedwith the effects of multiple genes in combination with lifestyle and environmentalfactors. Conditions caused by many contributing factors are called complex ormultifactorial disorders.

Although complex disorders often cluster in families, they do not have a clear-cutpattern of inheritance. This makes it difficult to determine a person’s risk of inheritingor passing on these disorders. Complex disorders are also difficult to study andtreat because the specific factors that cause most of these disorders have not yetbeen identified. By 2010, however, researchers predict they will have found themajor contributing genes for many common complex disorders.

For more information about complex disorders:

A fact sheet about the inheritance of multifactorial disorders(http://www.genetics.edu.au/Information/Genetics-Fact-Sheets/Environmetal-and-Genetic-Interactions-Complex-Patterns-of-Inheritance1-FS11) is available from theCentre for Genetics Education.

If you would like information about a specific complex disorder such as diabetes orobesity, MedlinePlus (http://www.nlm.nih.gov/medlineplus/) will lead you to factsheets and other reliable medical information. In addition, the Centers for DiseaseControl and Prevention provides a detailed list of diseases and conditions(http://www.cdc.gov/DiseasesConditions/) that links to additional information.

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What information about a genetic condition can statisticsprovide?

Statistical data can provide general information about how common a condition is,how many people have the condition, or how likely it is that a person will developthe condition. Statistics are not personalized, however—they offer estimates basedon groups of people. By taking into account a person’s family history, medicalhistory, and other factors, a genetics professional can help interpret what statisticsmean for a particular patient.

Some statistical terms are commonly used when describing genetic conditions andother disorders. These terms include:

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Common statistical termsExamplesDescriptionStatistical

term

About 1 in 200,000 peoplein the United States areborn with syndrome A eachyear. An estimated 15,000infants with syndrome Bwere born last yearworldwide.

The incidence of a gene mutation or agenetic disorder is the number of peoplewho are born with the mutation ordisorder in a specified group per year.Incidence is often written in the form “1in [a number]” or as a total number oflive births.

Incidence

Approximately 1 in 100,000people in the United Stateshave syndrome A at thepresent time. About100,000 children worldwidecurrently have syndrome B.

The prevalence of a gene mutation or agenetic disorder is the total number ofpeople in a specified group at a giventime who have the mutation or disorder.This term includes both newly diagnosedand pre-existing cases in people of anyage. Prevalence is often written in theform “1 in [a number]” or as a totalnumber of people who have a condition.

Prevalence

An estimated 12,000 peopleworldwide died fromsyndrome C in 2002.

Mortality is the number of deaths froma particular disorder occurring in aspecified group per year. Mortality isusually expressed as a total number ofdeaths.

Mortality

Approximately 1 percent ofpeople in the United Statesdevelop disorder D duringtheir lifetimes. The lifetimerisk of developing disorderD is 1 in 100.

Lifetime risk is the average risk ofdeveloping a particular disorder at somepoint during a lifetime. Lifetime risk isoften written as a percentage or as “1 in[a number].” It is important to rememberthat the risk per year or per decade ismuch lower than the lifetime risk. Inaddition, other factors may increase ordecrease a person’s risk as comparedwith the average.

Lifetime risk

For more information about understanding and interpreting statistics:

The New York Department of Health provides a basic explanation of statisticalterms (http://www.health.ny.gov/diseases/chronic/basicstat.htm), including incidence,prevalence, morbidity, and mortality.

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Information about interpreting cancer statistics (http://www.cdc.gov/excite/skincancer/mod04.htm) is available from the Centers for Disease Control and Prevention (CDC)as part of an educational module for students. Although this information focuseson cancer, information about health statistics can also apply to other disorders. TheNational Cancer Institute offers additional tools for understanding cancer statistics(http://www.cancer.gov/statistics/understanding).

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How are genetic conditions and genes named?Naming genetic conditions

Genetic conditions are not named in one standard way (unlike genes, which aregiven an official name and symbol by a formal committee). Doctors who treat familieswith a particular disorder are often the first to propose a name for the condition.Expert working groups may later revise the name to improve its usefulness. Namingis important because it allows accurate and effective communication about particularconditions, which will ultimately help researchers find new approaches to treatment.

Disorder names are often derived from one or a combination of sources:

• The basic genetic or biochemical defect that causes the condition (forexample, alpha-1 antitrypsin deficiency);

• One or more major signs or symptoms of the disorder (for example,hypermanganesemia with dystonia, polycythemia, and cirrhosis);

• The parts of the body affected by the condition (for example,craniofacial-deafness-hand syndrome);

• The name of a physician or researcher, often the first person to describethe disorder (for example, Marfan syndrome, which was named after Dr.Antoine Bernard-Jean Marfan);

• A geographic area (for example, familial Mediterranean fever, whichoccurs mainly in populations bordering the Mediterranean Sea); or

• The name of a patient or family with the condition (for example,amyotrophic lateral sclerosis, which is also called Lou Gehrig diseaseafter the famous baseball player who had the condition).

Disorders named after a specific person or place are called eponyms. There isdebate as to whether the possessive form (e.g., Alzheimer’s disease) or thenonpossessive form (Alzheimer disease) of eponyms is preferred. As a rule, medicalgeneticists use the nonpossessive form, and this form may become the standardfor doctors in all fields of medicine.

Naming genes

The HUGO Gene Nomenclature Committee (http://www.genenames.org/) (HGNC)designates an official name and symbol (an abbreviation of the name) for eachknown human gene. Some official gene names include additional information inparentheses, such as related genetic conditions, subtypes of a condition, orinheritance pattern. The HGNC is a non-profit organization funded by the U.K.Medical Research Council and the U.S. National Institutes of Health. The Committee

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has named more than 13,000 of the estimated 20,000 to 25,000 genes in the humangenome.

During the research process, genes often acquire several alternate names andsymbols. Different researchers investigating the same gene may each give thegene a different name, which can cause confusion. The HGNC assigns a uniquename and symbol to each human gene, which allows effective organization of genesin large databanks, aiding the advancement of research. For specific informationabout how genes are named, refer to the HGNC’s Guidelines for Human GeneNomenclature (http://www.genenames.org/guidelines.html).

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